<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>
<body>
  <script>
  var message = "Hello World"
  //1.修改字符串
  //1.严格的修改字符串，之前的字符串内部修改掉
  message[2] = 'a'
  console.log(message)
  //.toUpperCase()将所有的字符变成大写
  //.toLowerCase()将所有的字符变成小写
  
  //message中所有的字符变成大写
  //新生成的，原来的没有改变
  var message1 = message.toUpperCase()
  console.log(message1)
  
  </script>
</body>
</html>